1. Field of the Invention
The present invention relates generally to electron beam systems. The present invention more particularly relates to scanning electron microscopes in voltage contrast mode.
2. Description of the Background Art
A Scanning Electron Microscope (SEM) may be capable of Voltage Contrast inspection of a specimen (SEM-VC). SEM-VC inspection typically uses low landing energies and high electron current.
The specimen may be, for example, a semiconductor wafer with memory arrays thereon. The memory array may include, for example, three different types of contacts as illustrated in FIG. 1. FIG. 1 depicts examples of an N+ contact 102, a P+ contact 104, and a gate contact 106. The N+ contact 102 comprises a tungsten (W) plug to polysilicon over a N+ contact well within a surrounding P well. The P+ contact 104 comprises a tungsten plug to a P+ contact well within a surrounding N well. The gate contact 106 comprises a tungsten plug to a floating gate that is separated from an N or a P well by a thin gate oxide layer. The examples in FIG. 1 are formed on a P substrate. Alternatively, of course, such structures may be formed on an N substrate.
Consider, for example, the N+ contacts 102. A conventional retarding voltage may be applied between an (extracting) electrode above the specimen and the specimen stage. The retarding voltage causes electrons to build up on those tungsten plugs that are floating due to the N+ contact 102 being a defective open circuit. This is because the open circuit prevents the electrons from xe2x80x9cdraining awayxe2x80x9d from those tungsten plugs. The excess electrons at the surface of those tungsten plugs make them appear brighter in the SEM-VC retarding mode image. An example of this is illustrated in FIG. 2A. The brighter area in FIG. 2A is indicates the presence of open N+ contacts 102 in that area.
Alternatively, a conventional extracting voltage may be applied between the electrode above the specimen and the specimen stage. The extracting voltage causes electrons to be extracted from the tungsten plugs of normal N+ contacts 102. The extraction of electrons from a normal N+ contact 102, however, results in the N+ contact well becoming relatively positive charged. The positive charge of the N+ contact well causes the PN diode (from the surrounding P well to the N+ contact well) to be reversed biased. The reversed biased diode constrains and limits the flow of electrons out of the N+ contacts 102. The lesser flow of electrons the normal N+ contacts makes their tungsten plugs appear somewhat dim in the SEM-VC image. On the other hand, a shorted N+ contact 102 may have a short circuit that goes directly from the polysilicon to the P well (bypassing the PN reverse biased junction). Such a shorted contact would not have a reversed bias diode to constrain the flow of extracted electrons. Thus, a plug associated with a shorted N+ contact would appear brighter in an SEM-VC extraction mode image than would a plug associated with a normal N+ contact 102. An example of this is illustrated in FIG. 2B. The brighter area in FIG. 2B indicates the presence of shorted N+ contacts 102 in that area. Note that the image in FIG. 2B is roughly of the same area from the same memory as the image in FIG. 2A.
While conventional voltage contrast techniques are useful as described above, further improvement in defect detection techniques is desirable.